The vasa recta, the capillary networks that supply blood to the medulla, are highly permeable to solute and water. As with the loop of Henle, the vasa recta form a parallel set of hairpin loops within the medulla (see ). Not only do the vasa recta bring nutrients and oxygen to the medullary nephron segments, but more importantly they also remove the excess water and solute that is continuously added to the medullary interstitium by these nephron seg eOn a typical diet the kidneys must excrete 450 mmol/day of urea. At a maximal urine [urea] of 600 mmol/L this amount of urea can be excreted in less than 1 L of urine. However, if the maximal urine [urea] is reduced because of a decrease in the medullary interstitial fluid [urea], a larger urine volume would be needed to excrete the 450 mmol/day of urea (e.g., 2.25 L of urine would be required if the maximal urine [urea] was only 200 mM).
CHAPTER 35 Control of Body Fluid Osmolality and Volume ments. The ability of the vasa recta to maintain the medullary interstitial gradient is flow dependent. A substantial increase in vasa recta blood flow dissipates the medullary gradient (i.e., washout of osmoles from the medullary interstitium). Alternatively, reduced blood flow reduces oxygen delivery to the nephron segments within the medulla. Because transport of salt and other solutes requires oxygen and ATP, reduced medullary blood flow decreases salt and solute transport by nephron segments in the medulla. As a result the medullary interstitial osmotic gradient cannot be maintained.
Assessment of Renal Diluting and Concentrating Ability
Assessment of renal water handling includes measurements of urine osmolality and the volume of urine excreted. The range of urine osmolality is from 50 to 1200 mOsm/kg H2O. The corresponding range in urine volume is 18 L to as little as 0.5 L/day. These ranges are not fixed and vary from individual to individual. They can also be affected by disease processes, and as noted previously they are dependent on the amount of solute the kidneys must also excrete.
The ability of the kidneys to dilute or concentrate urine requires the separation of solute and water (i.e., the single effect of the countercurrent multiplication process). This separation of solute and water in essence generates a volume of water that is “free of solute.” When urine is dilute, solute-free water is excreted from the body. When urine is concentrated, solute-free water is returned to the body (i.e., conserved).
For the kidneys to maximally excrete solute-free water (i.e., 18 L/day) the following conditions must be met: 1.
AVP must be absent; without it the collecting duct does not reabsorb a significant amount of water.
2.
The tubular structures that separate solute from water (i.e., dilute the tubule fluid) must function normally. In the absence of AVP the following nephron segments can dilute the luminal fluid: ascending thin limb of Henle’s loop thick ascending limb of Henle’s loop
Because of its high transport rate, the thick ascending limb is quantitatively the most important nephron segment involved in the separation of solute and water.
3. An adequate amount of tubular fluid must be delivered to the aforementioned nephron sites for maximal separation of solute and water. Factors that reduce delivery (e.g., decreased GFR or enhanced proximal tubule reabsorption) impair the kidneys’ ability to maximally excrete solute-free water. Similar requirements also apply to conservation of water by the kidneys. For the kidneys to conserve water maximally (6–8 L/day) the following conditions must be met: 1. An adequate amount of tubular fluid must be delivered to those nephron segments that separate solute from water; the most important segment in this regard is the